1. Field of the Invention
Embodiments of the present invention generally relate to packaging integrated circuits, and more specifically, to build-up layers for package substrates.
2. Description of the Related Art
Package substrates are used in the packaging of integrated circuits to provide mounting surfaces for integrated circuits and to facilitate electrical connection therebetween. The package substrates include a plurality of electrically-conductive interconnects for transporting electric current. The interconnects are electrically insulated from one another by a dielectric material, also known as build-up layers, in which the interconnects are formed. Conventional package substrates, however, suffer from several drawbacks.
One conventional type of package substrate uses build-up layers formed from pre-impregnated composite fibers, also known as “pre-preg”. Pre-impregnated composite fibers, however, are not able to support high density wiring schemes, and therefore, limit the minimum size that can be obtained for a manufactured device. The formation of interconnects within pre-impregnated fibers generally includes the deposition of a metal foil over the pre-impregnated fibers, and subsequent removal of the undesired portions of the foil by etching. Such processes limit the interconnect densities that can be achieved.
Other conventional package substrates utilize Ajinomoto Build-up Film (ABF) for forming build-up layers. Due to the differences in composition between ABF and pre-impregnated fibers, ABF has different properties than the pre-impregnated fibers. ABF, while capable of supporting high density wiring schemes, is flexible and has a relatively high coefficient of thermal expansion. The high coefficient of thermal expansion increases the likelihood of undesirable bowing and cracking of the package substrate, thus resulting in damage to the integrated circuits coupled thereto.
Therefore, there is a need in the art for a package substrate having build-up layers capable of achieving finer interconnect geometries and increased package planarity.